Beginners' zoology . Fig. 56. — Water tubeSystem of starfish. in, madreporite; itc, stonecanal; ap, ampulla. 36 BEGINNERS ZOOLOGY •?XtVitiyvi^ - also called avibu/acral ieet (Latin ambulacra, forest walks).There is a water holder {ampulla), or muscular water bulb at the base of eachtube foot (), These con-tract and forcethe water intothe tube feet andextend cuplikeends of thetubes cling tothe ground bysuction. Thefeet contain delicate musclesby which theycontract andshorten. Thusthe animal pullsitself slowly along, hundreds of feet acting together. The tube feet, for theirown pr
Beginners' zoology . Fig. 56. — Water tubeSystem of starfish. in, madreporite; itc, stonecanal; ap, ampulla. 36 BEGINNERS ZOOLOGY •?XtVitiyvi^ - also called avibu/acral ieet (Latin ambulacra, forest walks).There is a water holder {ampulla), or muscular water bulb at the base of eachtube foot (), These con-tract and forcethe water intothe tube feet andextend cuplikeends of thetubes cling tothe ground bysuction. Thefeet contain delicate musclesby which theycontract andshorten. Thusthe animal pullsitself slowly along, hundreds of feet acting together. The tube feet, for theirown protection, may contract and retire into the groove, thewater which extended them being sent back into the system of water X ^. Fig. 57. —Starfish, from below; tube feet extended. Stei vessels (or water-vascular system) ofthe echinodermatais characteristic ofthem; is notfound elsewhere inthe animal grooves and theplates on each sideof them occupy thea7nbulacral areas. The rows of spines on each side of the groovesare freely movable. (What advantage ?) The spines on the abcralsurface are not freely movable.
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1921
